Hepatic peroxisomes contain about 33 to 45% of the fatty acid beta oxidation capacity of adult rat or mice livers and the rest in the mitochondria. Changes in the distribution of fatty acid oxidation between these two organelles during postnatal development, during starvation, and during other treatments such as by hypolipidemic agents is under investigation. Both peroxisomal and mitochondrial activities develop postnatal. The regulation of both by diet will be examined. Preliminary data indicate that more rapid fluctuation in the mitochondria activity occurs than in the peroxisomal fraction. Associated with fatty acid oxidation in the peroxisomes is a glycerol-phosphate shuttle which appears to carry reducing capacity from the peroxisomes so that beta oxidation can continue. Further elucidation of this alternate fatty acid beta oxidation system will be continued. The effect of obesity, diet, and drugs on the distribution of two fatty acid systems between the peroxisomes and mitochondria will also be continued.